basically, it gives you the ability to combine them  an image editor and optimize how you want them to look.  D|S has a tool that does this (you can even render specular alone), and, iirc, Dreamlight, theSea and others have shown how to use the layers for optimal drama or realism.  i never got into it myself and didn't feel like spending $14 on making my postwork even more complex (because every figure has major anatomical flaws obvious even to me).  i had no clue it was in Poser (probably not P6, though, huh?).  i think i'd have to majorly upgrade my computer along with my version of Poser to try something like that. my final renders are 2400+ px.  but maybe separating each aspect out would be faster than i'd think.